1. Field of the Invention
The present invention relates to a service system using a cable television (CATV) network, and more particularly, to a reverse signal transmitting device.
2. Description of the Related Art
A variety of additional services have recently been provided over a CATV network in addition to multi-channel broadcasting. For this purpose, a plurality of subscribers are grouped into a cell and a predetermined number of cells are again grouped into a zone in a system using a HFC (Hybrid Fiber and Coaxial) network as a CATV network.
Signal transmissions for a service over a CATV network are largely divided into forward transmission and reverse transmission. The forward transmission refers to transmission of a signal (forward signal) for services from a service provider to a subscriber, and the reverse transmission indicates transmission of a signal (reverse signal) for requesting for services from a subscriber to a service provider. See, for example, U.S. Pat. No. 4,367,557 to Joseph L. Stern et al. entitled Wired Broadcasting Systems; U.S. Pat. No. 5,790,523 to John H. Ritchie, Jr. et al. entitled testing Facility For A Broadband Communications System; and U.S. Pat. No. 5,826,167 to Catherine W. Jelinek et al. entitled Bi-Directional Cable Television System Including A UHF Filter, incorporated by reference herein.
Except for distortion of a forward signal during transmission, there is no other great difficulty in the forward transmission since a single sender, that is, a single service provider distributes the forward signal to a plurality of subscribers. On the other hand, the reverse transmission may make it impossible to recover an original reverse signal since reverse signals from a plurality of subscribers are combined in the course of heading for one destination, namely, the service provider in view of characteristics of the RF (Radio Frequency) signals. Therefore, a reverse signal is transmitted at a different frequency assigned to each subscriber or in time division under the control of the service provider, for example, in a polling scheme. In particular, when a plurality of subscribers use different frequencies in sending their reverse signals to the service provider, noise introduced into a reverse signal may adversely affect other reverse signals.
This will be described referring to FIG. 1 illustrating connections between a service provider and subscribers in a conventional service system using a CATV network. In FIG. 1, first to Nth cells C1 to CN each accommodate a plurality of subscribers and belong to zone A. The subscribers within zone A are connected to the service provider through a head end (or headend) 100. Reverse signals from the subscribers within an identical cell and then an identical zone are combined prior to transmission to the service provider through the head end 100. The service provider transmits a forward signal to the subscribers through the head end 100.
This conventional service system has a distinctive shortcoming that if noise is introduced into a reverse signal from a subscriber within the first cell C1, the noise influences reverse signals from other subscribers during combination of the reverse signals. As a result, all the combined reverse signals cannot be recovered.